1. Field of the Invention
The present invention relates to an optical pickup apparatus executing a reading-out operation of a signal recorded in an optical disc or a recording operation of a signal in the optical disc.
2. Description of the Related Art
There has been widely available an optical disc device capable of executing the signal reading-out operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of the optical disc.
While there are prevailing in general the optical disc devices using optical discs called CD or DVD, there have recently been developed the optical disc devices using optical discs whose recording density is improved, i.e. Blu-ray standard, etc.
Infrared light with a wavelength of 780 nm is used as the laser light executing the reading-out operation of the signal recorded in the CD standard optical disc, and red light with a wavelength of 650 nm is used as the laser light executing the reading-out operation of the signal recorded in the DVD standard optical disc.
Thickness of a protective layer provided on an upper face of the signal recording layer in the CD standard optical disc is 1.2 mm, and the numerical aperture of an objective lens used for the reading-out operation of the signal from this signal recording layer is specified at 0.45. The thickness of the protective layer provided on the upper face of the signal recording layer in the DVD standard optical disc is 0.6 mm and the numerical aperture of the objective lens used for the reading-out operation of the signal from this signal recording layer is specified at 0.6.
In contrast to such CD-standard and DVD-standard optical discs, laser light with a short wavelength, or blue-violet light with a wavelength of 405 nm, for example, is used as the laser light carrying out the reading-out operation of the signal recorded in the Blu-ray standard or HD DVD standard optical discs.
The thickness of the protective layer provided on the upper face of the signal recording layer in the Blu-ray standard optical disc is 0.1 mm, and the numerical aperture of the objective lens used for the reading-out operation of the signal from this signal recording layer is specified at 0.85.
For a reproduction operation of the signal recorded in the signal recording layer included in the Blu-ray standard optical disc or for a recording operation of the signal in the signal recording layer, a diameter of a laser spot generated by focusing laser light needs to be reduced. In the objective lens used to obtain the desired laser spot shape, not only the numerical aperture (NA) thereof is increased but a focal length thereof becomes shorter and, therefore, a radius of curvature of the objective lens becomes smaller.
In the optical pickup apparatus, a laser diode is used as an element for generating the laser light to be emitted, but intensity of the laser light emitted from the laser diode is not uniform and has a property called the Gaussian distribution. The intensity distribution of such a laser light is in an elliptical shape as known in the laser diode.
In the optical pickup apparatus using the objective lens having the small radius of curvature, since the elliptical laser light emitted from the laser diode enters the objective lens, there is decreased rim intensity, i.e., the intensity of the laser light passing through a peripheral portion of the objective lens relative to the intensity of the laser light passing through in the vicinity of an optical axis thereof. As a result, there exists a problem not only that the peripheral portion of the laser spot is blurred but that a focusing intensity is decreased.
In the case of a lens having a small radius of curvature and a large numerical aperture such as the objective lens as mentioned above, an incident angle of the laser light is increased on the peripheral side of the objective lens, i.e., at a position where the numerical aperture is large, thereby increasing reflectance, so that the above-mentioned rim intensity decreases due to the above properties.
There is proposed a method of forming an antireflective film on the objective lens as a method for solving such a problem to obtain a desired laser spot (See Japanese Patent Application Laid-Open Publication No. 2004-39161).
In the above Patent Document, there is described an art for minimizing reflectance at a predetermined position by the antireflective film provided on an objective lens. Although the rim intensity can be improved with such an art, it can be applied only when the intensity is uniform of the laser beam entering an entrance pupil of the objective lens.
The objective lens is made up so as to be displaced in a radial direction of the optical disc under a tracking control operation, as is well known. Therefore, the intensity becomes nonuniform of the laser beam entering the entrance pupil of the objective lens due to displacement of the objective lens under the tracking control operation.
FIGS. 5A and 5B show a relation between an entrance pupil S and light intensity on the objective lens in the optical pickup apparatus. FIG. 5A shows a case where the objective lens is at a neutral position, that is, a position where the objective lens is not displaced by the tracking control operation, and FIG. 5B shows a case where the objective lens is at a position where the objective lens is displaced in a tracking direction by the tracking control operation.
In FIGS. 5A and 5B, a range e represented by dots shows a portion in which the light intensity is low. FIG. 5A shows, as mentioned above, a case where the objective lens is at the neutral position, and if the light intensity on the entrance pupil S of the objective lens is not uniform, when the objective lens is displaced in the tracking direction, the light intensity on the entrance pupil S of the objective lens becomes greatly nonuniform in the tracking direction as shown in FIG. 5B.
If the intensity in the tracking direction of the laser beam entering the entrance pupil becomes nonuniform as the objective lens is displaced in the tracking direction, the rim intensity in the tracking direction decreases, so that the shape of a spot of the laser beam focused to be formed on the signal recording layer of the optical disc becomes larger. As a result, there are problems that reproduction characteristics of the signal recorded in the optical disc and recording characteristics of the signal in the optical disc are deteriorated.